All the usual vendors sell them. Snyders, Bratton's etc. Most of it is made by howell's in Texas. What ever you do do not buy direct from Howell's. You will pull all your hair out dealing with that Ass#$)%$. You can buy direct if you are going to pick up but it takes him months to ship. His stuff is OK but you like all of them you need to massage it some to make it fit correctly. You should try Bert's in Denver. I understand he sells original sheet metal and has a pretty good supply of it.
that covers a lot of ground from the extremely(1904)to the time you determine as being old as nick said.if as noted if you order from howell's you will be howling in anger(play on words)
The Jalopy Journal
